Zinc

Zinc, commonly found as a divalent cation (Zn2+), functions as a cofactor to enzymes that contribute to virtually all functions in the body.¹

Absorption: Primarily in the jejunum of the small intestine, zinc uses a carrier-mediated system that goes against the concentration gradient but is thought to not use ATP.¹ It is believed that the primary method for absorption is a specific zinc transporter, transporter ZIP4.¹ Transporters DMT1  and ZIP14 also bind to zinc. Once in the enterocyte, zinc can perform functions within, exit into the bloodstream, or bind to the protein metallothionein to be excreted.¹ Metallothionein can bind to copper as well, leading to sequential loss.¹ Within the intestinal cell, zinc is stored within various organelles regulated by various importer and exporter proteins. Zinc transporter 1 (ZnT-1) moves zinc from the cytoplasm to the blood based on physiological needs.¹ Paracellular diffusion is also a method of zinc absorption into and out of intestinal cells.

Zinc absorption is affected by a variety of factors. In soy and wheat products, zinc absorption is enhanced by the presence of casein, cysteine, or histidine.¹ Phytates (in the presence of calcium), oxalates, polyphenols, and fiber found in foods can decrease absorption. Iron also competes with zinc, lowering the absorption of the mineral.

Storage and Excretion: Zinc is primarily found in skeletal muscle and bone, but it is also found in relatively high concentrations within the liver, brain, kidneys, and heart. Zinc uses a selective transport system to transfer it within tissues from blood.¹ Zinc is primarily excreted in feces but also by way of sweat and urine. 

Functions:  

Zinc is part of over 200 enzymes responsible for a variety of functions and gene expression. 

Enzymes - As a cofactor for a variety of enzymes, roles include pH regulation, bone mineralization, alcohol metabolism, heme synthesis, protein/nucleic acid metabolism, digestion, and more. Examples of zinc-dependent enzymes are carbonic anhydrase, alkaline phosphatase, alcohol dehydrogenase, gustin, etc.¹

Gene Expression - Zinc binds with amino acids in proteins known as transcription factors that are responsible for gene expression. Zip and Zn proteins, regulators of zinc, are upregulated or downregulated depending on need.¹

Dietary Reference Intake (DRI) - The recommended dietary allowance (RDA) for zinc is 11 milligrams for men and 8 milligrams for women 19 years old and older.²  The tolerable upper limit for adult men and women (19 years and older ) is 40 milligrams (mg).² 

Deficiency/Toxicity- Zinc deficiency is not common in the U.S. compared to Middle Eastern populations due to the regular consumption of unleavened bread (phytates bind to zinc) and low meat consumption.¹ Zinc deficiency affects a variety of functions within the body and can lead to dwarfism, deformed bones, poor wound healing, G.I. issues, and central nervous system abnormalities among other symptoms.¹

Zinc toxicity is uncommon but can lead to microcytic anemia through decreased copper and iron absorption, and hemolytic anemia can also occur.¹

Dietary Sources of Zinc 

Zinc is found in a variety of foods but is prevalent among meat, poultry, fish, and animal products. Fruits are not a good source of zinc. As previously stated, zinc absorption can decrease based on phytates, oxalates, polyphenols, and fibers found in foods.¹ It competes with iron. Other foods containing zinc include dairy products, grains, and legumes.

Examples of foods containing zinc include

Meat/Seafood/Animal Products- oysters, beef, pork, chicken, egg 

Dairy - milk, cheddar cheese 

Legumes - lentils, beans 

Grains - cereals, oatmeal, rice 

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

2. https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional/